dissecting aorta
DEFINITION
Clotted blood within the damaged aortic tunica media is termed “intramural hematoma,” whereas “aortic dissection” denotes one or more tears between the aortic lumen and a medial cleavage plane containing active blood flow.
Aortic dissection is differentiated from spontaneous rupture of the aortic wall or laceration of the intima and media without or with only limited separation of wall layers.
Aortic dissection may be localized to the portion of the aorta where the blood first enters the wall (primary entry, primary tear), but usually involves most or all of the vessel.
The dissection often includes side branches, particularly supra-aortic branches and pelvic vessels of the elastic type.
The dissection rarely extends completely around the circumference; usually a strip of aortic wall remains intact. The separation often progresses distally from the primary entry but may propagate a variable distance proximally (retrograde dissection).
One or more fenestrations or re-entry tears often are present to provide communication channels between the true and false lumens of the double barrel aorta .
The true and the false channels are separated by the dissecting or intimal membrane or flap that contains aortic media and intima.
The dissection may stop at the most distal re-entry site or may end in a downstream blind pocket.
Blood within blind pockets, whether created by antegrade or retrograde dissection, commonly thromboses.
Blood flow in the true (intimal-lined) aortic channel or dissected side branches may be compromised by compression by the false lumen to cause malperfusion, or ischemia of downstream organs and extremities
Because the dissection splits the outer layers of the media and weakens the external aortic coat, the wall of the false channel may either rupture or dilate to ultimately produce an aneurysm.
The older term, dissecting aneurysm, is only justified when aneurysm develops or if the dissection involves a pre-existing aortic aneurysm.
The false channel eventually develops an endothelial lining, but may contain extensive thrombus.
The onset of an aortic dissection almost always can be traced to an episode of severe chest or back pain. Arbitrarily, the acute stage is terminated at 14 days. The subsequent period that extends to 2 months after the primary event is termed subacute; the chronic phase begins after the second month.
CLASSIFICATION
There are two accepted classifications of aortic dissection
. DeBakey defines a dissection beginning in the proximal aorta that involves most of or the entire vessel as type I. Type II involves only the ascending aorta.
Dissection of the descending thoracic aorta is termed type III. Type III dissections are subdivided into IIIa, which is limited to the proximal descending portion, and subtype IIIb, which extends beyond the diaphragmatic hiatus into the suprarenal and abdominal aorta.
At Stanford University, Daily considers any dissection involving the ascending aorta an indication for emergency surgical intervention. This variant is termed type A regardless of the site of origin or the distal extent of the process. Descending thoracic aortic dissections are labeled type B
. Unfortunately, grouping DeBakey's type I and II classifications together ignores very different operative and long-term outcomes associated with these different entities.
The classification of dissection used in this text is that advocated by Kirklin. Proximal aortic dissection involves the intrapericardial ascending aorta and may include the aortic arch. Distal aortic dissection begins beyond the left subclavian artery but may include a retrogradely dissected portion of the arch. However, if the retrograde dissection extends proximally into the ascending aorta and involves a portion of the intrapericardial segment, the dissection is labeled proximal. Thus Stanford type A and DeBakey types I and II and type III with retrograde extension into the intrapericardial aorta are included in the proximal dissection category. The distal dissection category includes DeBakey type III and Stanford type B.
Classification of aortic dissection. DeBakey type I and Stanford Class A include dissections that involve the proximal aorta, arch, and descending thoracic aorta. DeBakey type II only involves the ascending aorta; this dissection is included in Stanford Class A. DeBakey type III and Stanford Class B include dissections that originate in the descending thoracic and thoracoabdominal aorta regardless of any retrograde involvement of the arch. These are subdivided into a and b depending on abdominal aortic involvement
Classification of aortic dissection. DeBakey type I and Stanford Class A include dissections that involve the proximal aorta, arch, and descending thoracic aorta. DeBakey type II only involves the ascending aorta; this dissection is included in Stanford Class A. DeBakey type III and Stanford Class B include dissections that originate in the descending thoracic and thoracoabdominal aorta regardless of any retrograde involvement of the arch. These are subdivided into a and b depending on abdominal aortic involvement
ETIOLOGY
The term cystic medial necrosis or medial degeneration is no longer considered the common structural disorder of aortic dissection. Severe derangement and loss of elastic tissue are typical of dissection in younger patients with congenital connective tissue disorders, but correlate poorly with dissections in other patients.
Anagnostopoulos described seven general conditions associated with damage to the aortic media.
These include hypertension and hypervolemia, interference with the assembly and maintenance of the media, destruction by mechanical forces, or chemical agents, obstruction to aortic flow, and elevated cardiac output.
Several heritable disorders of elastic tissue predispose the aorta to early dissection. These include Marfan, Turner, Noonan, and Ehlers-Danlos syndromes.
A fundamental genetic derangement in the assembly and deposition of a newly discovered microfibrillar protein, fibrillin, occurs in Marfan syndrome.
Both sino-tubular ectasia and dissection may occur in non-Marfan families as a heritable autosomal dominant defect.
In such patients, the aortic valve annulus may be normal or dilated and distorted.
Unicuspid and bicuspid valve syndromes are associated with aortic dissection.
In Edwards' series of 119 fatal dissections, 9 percent had congenitally abnormal aortic valves.
Roberts confirmed this incidence.
The rate of dissection in patients with congenitally deformed aortic valves is five times that of trileaflet valves and is highest with unicuspid valves. Isolated stenosis of a three-leaflet valve is not considered a predisposing factor for aortic dissection.
As first noted in Abbott's series of 200 cases, coarctation of the aortic isthmus predisposes the aorta to dissection, particularly in older patients.
In the absence of a connective tissue disorder, arterial hypertension is the most important predisposing factor for aortic dissection. Most large autopsy and clinical series estimate an incidence of hypertension in patients with dissection to be at least 75 percent.
Hypertension causes excessive mechanical and metabolic strain on the media, which opposes arterial pressure within the aortic wall. The causative role of hypertension is underlined by the rare event of pulmonary artery dissection that only occurs with pulmonary hypertension.
Pregnancy is associated with half of the dissections that affect females below the age of 40 and usually occurs in the last trimester or during labor.
Contributing factors include hypercirculation during the late gestational period, hypertension, and the loosened connective tissue owing to hormonal changes.
Most authors believe atherosclerosis is coincidental rather than causative of aortic dissection.
Severe atherosclerosis in infra-renal aneurysms actually may impede dissection. Unfortunately, aortic dissection of an arteriosclerotic aneurysm increases the risk of rupture.
Extensive dissection beyond a traumatic aortic tear rarely occurs. The majority of traumatic aortic lacerations are limited to the aortic isthmus and occasionally may cause partial or circular prolapse of intima and media into the distal vessel to produce pseudocoarctation.
Iatrogenic trauma occasionally causes aortic dissection. Radiological diagnostic procedures coronary angiography, and therapeutic balloon catheter manipulation all have produced dissections. Insertion of an intra-aortic balloon catheter risks vascular injury—usually occlusion or perforation of a pelvic vessel. However, extensive type B or even retrograde type A dissection does occur.
Femoral arterial cannulation for retrograde extracorporeal perfusion is notorious for producing dissection of pelvic vessels and sometimes the aorta.
Cannula insertion or the impact of the high-pressure blood jet emanating from the cannula may create a false passage within the vascular wall.
Antegrade cannulation of the ascending aorta or arch likewise may create a false passage within the medial layer or may disrupt the aortic wall opposite the cannulation site to trigger dissection.
This risk is increased when the ascending aorta is dilated.
Local aortic trauma caused by a cross-clamp or side-biting clamp is another well-known source of aortic dissection.
Operations involving the ascending aorta, such as aorto-coronary vein bypass grafting and aortic valve replacement are associated with dissection.
Rarely, aortic dissection is associated with other conditions such as hypercortisolism, pheochromocytoma, lupus erythmatosus, chronic nephropathic cystinosis, and osteogenesis imperfecta.
More recently, aortic dissection occurred after cocaine use.
HISTOLOGY AND STRUCTURE
The aortic wall is composed of three layers, the intima, the tunica media, and the adventitia. The media is the strongest component of the aortic wall and measures on average 1.2 mm in the ascending vessel and is the layer most affected in aortic dissection. It is composed of elastic and collagen fibers, that each account for 20–30 percent of the aortic wall by volume; smooth muscle cells represent 5 percent. Smooth muscle cells produce all the constituents of the medial layer. Elastic sheets are concentrically arranged fenestrated lamellae with intertwining individual fibers. Interconnecting fibers hold the layers together. Microfibrils, recently discovered, contain the glycoprotein fibrillin and are an important constituent of the aortic media. Microfibrils provide a scaffold for deposition of elastin and become completely encased with elastin to produce concentric rings of the tunica media. Derangement of these microfibrils because of a base pair substitution is the base deficiency responsible for Marfan syndrome. Kainulainen in 1990 and Dietz in 1992 mapped the genetic defect of Marfan syndrome to the long arm of chromosome 15 (15q).
ACUTE AORTIC DISSECTION
Clinical Presentation
Although a small proportion of patients succumb suddenly after acute aortic dissection, the majority stabilize for a brief but definite interval. This period can be extended with appropriate medical intervention. As with all catastrophic surgical problems, the initial prognosis depends on the severity and extent of the dissection, the overall physical condition and age of the patient, the experience and expertise of the team dealing with the problem, and the rapidity with which the problem can be addressed.
The most important factor leading to a successful early diagnosis of acute aortic dissection is a high index of suspicion by the examining physician. Most patients with acute aortic dissection experience sudden severe chest pain, marked anxiety, and are aware that a catastrophic illness has occurred. A history of severe hypertension or known thoracic aortic aneurysm, of course, should alert the examiner to the possibility of aortic dissection. Tall stature, sternal deformity, and other characteristic stigmata of Marfan syndrome in the presence of chest pain should particularly prompt an examiner to rule out aortic dissection.
Severe chest pain is the cardinal feature of aortic section and occurs in more than 90 percent of patients. [
When the proximal aorta is involved, symptoms are characteristically centered in the mid-substernal area; patients with distal dissection tend to experience primarily interscapular pain. With DeBakey type I dissection, as the hematoma propagates from proximal to distal aorta, the pain may move from the anterior chest to the neck, to the interscapular area, and eventually involve much of the mid-back,
This migratory pain pattern should arouse suspicion of aortic dissection.
Chest pain caused by acute aortic dissection must be differentiated from that caused by coronary ischemia, myocardial infarction, and other thoracic diseases that include aortic regurgitation without dissection, aortic aneurysm without dissection, musculoskeletal pain, pericarditis, biliary colic, and pulmonary embolus. The pain of dissection is usually sudden, worst at onset, severe, and not previously experienced. Adjectives such as “ripping” and “tearing” often are used by the patient.
Usually the pain of dissection is constant as compared to the pain of cardiac ischemia, which may build slowly over time to a crescendo and improve with rest.
Because suspected acute coronary ischemia may be treated immediately with thrombolytic agents, differentiation of the two diseases must be made promptly and conclusively. Thrombolytics may be lethal for aortic dissection.
Total or partial ostial coronary artery occlusion by an aortic root dissecting membrane may complicate the diagnosis by producing angina or ischemic cardiac failure.
Occlusion of the thoracoabdominal aorta by the dissecting membrane may cause abdominal pain caused by renal or visceral ischemia. Similarly, iliac occlusion may produce severe peripheral ischemia and painful extremities.
Rarely, painless aortic dissection occurs in patients with pre-existing, large ascending aortic aneurysms.
PHYSICAL FINDINGS
Patients with acute dissection may appear pale and clammy but blood pressure is usually elevated because of essential hypertension or mechanical occlusion of a renal artery or the aorta
Pain and anxiety usually increase circulating catecholamines.
Hypotension or profound shock associated with aortic dissection usually indicates pericardial tamponade or manifest rupture when the proximal portion is involved or hemorrhage into the retroperitoneum or thoracic cavity in distal dissection. Other sources of hypotension include obstruction of a main coronary artery(s) and severe acute aortic insufficiency.
A new pulse deficit is a critical finding that supports the diagnosis of acute aortic dissection and occurs in up to 60 percent of patients with acute proximal dissection.
A pulse deficit involving either subclavian artery usually denotes proximal dissection, but distal dissection with retrograde extension may occlude the left subclavian artery.
Femoral pulses may be lost because of occlusion of either the thoracoabdominal aorta or iliac arteries from an expanding false lumen or, more commonly, dissection into one of the pelvic vessels. A pulse deficit can change as the hematoma dissects distally, expands, and re-enters, a finding extremely supportive of the diagnosis of aortic dissection.
The characteristic murmur of aortic regurgitation is present in most patients with acute proximal aortic dissection and in a minority of those with distal dissection, because of pre-existing annular dilatation without dissection.
A new murmur of aortic insufficiency with chest pain and a pulse deficit strongly suggests proximal dissection. Other auscultatory findings may include an S3 gallop owing to severe aortic regurgitation, a pericardial friction rub, or pulmonary edema. Loss of alveolar breath sounds in the left chest suggests hemothorax.
Signs of pericardial involvement such as jugular venous distension or a paradoxical pulse suggest pericardial involvement and impending tamponade.
Occasionally acute dissection causes neurologic deficits, ischemic paralysis, paraplegia, or Horner syndrome. Syncope is particularly ominous and may indicate rupture into the pericardium.Stroke may be owing to acute occlusion of a carotid artery that rarely improves with restoration of flow.
Furthermore, reperfusion may produce severe intracerebral hemorrhage, swelling, global cerebral injury, coma, and brain death.
Obstruction of intercostal and lumbar arteries from shearing off or compression by the dissecting membrane may result in permanent flaccid or spastic motor paraplegia.
This must be differentiated from lower body and peripheral nerve ischemia owing to occlusion of the thoracoabdominal aorta because restoration of perfusion to ischemic lower extremity muscles and peripheral nerves usually restores function.
Rarely expansion of the false lumen and compression of adjacent structures may cause the superior vena caval syndrome, vocal cord paralysis and hoarseness, tracheal or bronchial compression with attendant pulmonary collapse, massive hemoptysis from erosion into the tracheo-bronchial tree, or hematemesis from erosion into the esophagus.
Degradation of clotting blood or release of pyrogenic substances from ischemic tissues can produce low-grade fevers.
DIAGNOSTIC STUDIES
Severe chest pain without electrocardiographic (ECG) changes is the sine qua non of acute aortic dissection.
In acute proximal dissection, the ECG usually shows no ischemic changes. However, obstruction of a coronary ostium by the dissection causes significant S-T segment changes because of severe ischemia or infarction. Underlying coronary artery disease or hypertension also may leave evidence of previous myocardial infarction or hypertrophy.
Although standard chest x-rays are not diagnostic, several findings are strongly associated with aortic dissection.
In an asymptomatic patient, the chest x-ray may provide the first evidence of aortic pathology. Findings suggestive of aortic dissection include slight bulging of the descending thoracic aorta, deformity of the aortic knob, a density adjacent to the brachio-cephalic artery, enlargement of the cardiac shadow, displacement of the esophagus after passage of a nasogastric tube, widened mediastinum, irregular aortic contour, loss of sharpness of the aortic shadow, tracheal or bronchial displacement, and pleural effusion. Over 80 percent of patients with aortic dissection have one or more plain chest x-ray abnormalities, but a small number have completely normal evaluations. Thus, a normal chest x-ray does not rule out dissection. Serial x-rays may show enlargement of the aortic shadow or development of a pleural effusion. Small pleural effusions, usually on the left side, are not unusual after an aortic dissection and generally do not indicate rupture.
However, large effusions may be associated with widening of the mediastinal shadow from mediastinal bleeding or aortic expansion and signify rupture into the pleural space.
BLOOD TESTS
Blood tests usually are unremarkable in acute aortic dissection. A mild anemia is common because of sequestration of blood in the false lumen even without hemorrhage. Hemolysis of static blood may slightly increase bilirubin and lactic acid dehydrogenase. White blood cell counts between 10,000–15,000 are common.
Transaminases are normal or mildly elevated unless a malperfusion syndrome produces hepatic ischemia.
Ischemic tissue may produce a mild metabolic acidosis, but otherwise electrolytes usually are normal.
Initial Management
If prior information or impressions raise the possibility of acute aortic dissection, initial management in an intensive care unit (preferably surgical) often expedites evaluation, instrumentation, diagnosis, and preparations for operation. The history and physical examination are done simultaneously with insertion of a large-bore intravenous catheter and removal of a blood sample for blood typing, complete blood count and electrolytes, creatinine, urea nitrogen, and glucose. If systolic blood pressure is over 100 mmHg, medical therapy is started immediately (see the following) as rupture may occur during the evaluation process.
The initial work-up focuses on the differential diagnosis of acute aortic dissection. An electrocardiogram is obtained and the ECG is monitored continuously. A radial or brachial arterial catheter and a Swan-Ganz catheter are inserted. A Foley catheter is placed to monitor urine output; urine volume equal to 1 mL/kg/h indicates adequate renal perfusion. A portable AP chest x-ray is taken.
Symptoms, neurologic status, strength of peripheral pulses, and continuously monitored signals are checked frequently for changes during the diagnostic evaluation. A definitive diagnostic imaging modality is selected depending on the information available and hemodynamic stability of the patient. Often the patient is transferred to the operating room for transesophageal echocardiography after induction of general anesthesia.
Uncomfortable procedures such as angiography or transesophageal echocardiography increase heart rate and blood pressure and rupture has occurred during both procedures.
PHARMACOLOGIC THERAPY
The landmark review by Wheat and colleagues described the pharmacologic reduction of aortic wall stress in the setting of acute aortic dissection.
Initially Wheat and associates used reserpine, trimethaphane, and guanethidine.
Later, drug treatment was tailored to control both arterial hypertension and the force of left ventricular ejection, dP/dt, using vasodilators and beta-adrenergic blockade. Hypertension and the force of the pressure-pulse wave entering the aorta are the factors responsible for propagation and rupture of the dissection.
Sodium nitroprusside, a potent and short-acting vasodilator, is the drug of first choice for antihypertensive treatment in acute aortic dissection. The infusion is adjusted to maintain a systolic blood pressure between 90–100 mmHg. Metabolism of nitroprusside produces thiocyanate and cyanide toxicity may develop after continuous use over several days. Because sodium nitroprusside reduces afterload, the resultant increase in dP/dt may propagate a dissection; therefore, beta-adrenergic blocking agents are administered simultaneously.
Beta-adrenergic blocking drugs decrease the force of left ventricular ejection dP/dt max in acute dissection.
Short-acting compounds are most frequently employed, particularly if cardiopulmonary bypass is anticipated. Esmolol is a beta-1-selective adrenergic receptor blocking agent with an ultra-short duration of action. After a loading dose it reaches a steady state within 6 minutes; however, treatment without a loading dose is recommended. The half-life is approximately 9 minutes. The predictable rapid onset and short half-life of esmolol makes it ideal for acute aortic dissection. The drug is successfully combined with sodium nitroprusside but can induce sudden heart block.
Propranolol is administered in small boluses after an initial test dose of 0.25–0.5 mg. Repeat dosage, up to 1–2 mg, should not be administered at intervals less than 10 minutes. A reduction in pulse rate to 60 or 70 per minute indicates satisfactory beta blockade that can be maintained by re-dosing every 4–6 hours. A more cardioselective and longer acting beta-1-receptor blocker is atenolol. Intravenous dosage is administered slowly up to 0.15 mg/kg/d.
Labetalol is an ideal drug for use in aortic dissection because it has both alpha- and non-selective beta adrenoreceptor–blocking actions. A dose-dependent decrease in maximum dP/dt with a concomitant reduction in arterial blood pressure usually is achieved within 5 minutes of administration. Unfortunately, the drug's half-life, which approaches 24 hours, can complicate management of patients who require operation.
Pharmacologic therapy is started as soon as possible after the diagnosis of acute aortic dissection is suspected. Systolic blood pressure usually is maintained between 100–110 mmHg, but is reduced further if pain is not relieved as long as the patient remains conscious and urine output is adequate.
Diagnostic Imaging
The definitive diagnosis of aortic dissection requires identification of the dissection membrane that separates the aorta into true and false lumens. Modern imaging technology, including aortography, computerized axial tomography (CT), echocardiography, and magnetic resonance imaging (MRI) can demonstrate the presence or absence of the dissection membrane. Each imaging technique has advantages and disadvantages according to the clinical situation.
AORTOGRAPHY
In 1952, Halliday described the angiographic hallmarks of aortic dissection that are still valid today. These include opacification of a false lumen compressing the true lumen with a line of demarcation (the dissecting membrane) between the two channels
. Because aortography specifies flowing blood, angiograms detect an intramural hematoma but may fail to detect the false lumen if blood flow is brisk or if both channels are heavily opacified.
Coronary angiography prior to emergency repair of an acute proximal aortic dissection is not recommended. Even in the presence of moderate coronary artery disease, early repair of the dissection has precedence over other procedures. Although involvement of one or more of the main coronary arteries occurs in approximately 10–30 percent of patients with ascending aortic dissection, selective coronary angiography has little or no role in the diagnosis and management of this problem.
The ostial coronary anatomy and pathology is easily assessed intraoperatively.
In a personal series of 27 patients with acute dissection involving the proximal aorta, 16 had an electrocardiogram suggestive of coronary artery disease. Of these, five underwent preoperative cardiac catheterization but none of the 16 required coronary artery bypass surgery or died. Six of the remaining 11 patients with no clinical history of coronary artery disease and normal electrocardiograms had cardiac catheterization, but none had coronary artery disease. A single patient, with a history of coronary disease, who did not undergo cardiac catheterization, suffered a perioperative myocardial infarction.
CT SCANNING
Harris reported the first diagnosis of aortic dissection using computerized axial tomography in 1979. Definitive diagnosis requires identification of two or more channels separated by a dissecting membrane . CT scans are superior to aortography in both sensitivity and specificity, particularly if flow in both channels or flow in the false lumen is reduced or clotted. Contrast material is needed but must not overopacify and obscure the two lumens. Spiral scanning with 2- and 3-D reconstruction of the aorta provides spectacular images in less than 10 minutes and rivals those obtained from magnetic resonance imaging
Furthermore, CT scans are increasingly available around the clock, and are easily interpretable.
MAGNETIC RESONANCE IMAGING (MRI)
Magnetic resonance imaging provides similar information to that obtained from CT scanning. MRI is based primarily on the excitation of protons contained in water, and analysis of the radiofrequency pulse emitted as the proton relaxes.
The emitted signal produces an image based on proton density and pulse energy. In the aortic lumen, water and its excited protons move quickly through imaged areas with the flow of blood. Thus, with typical spin echo, MRI intra-arterial spaces appear completely black because excited protons have moved on prior to relaxation and pulse emission. Conversely, the vessel wall, dissecting membrane, and surrounding tissues and organs produce variable amounts of signal based on their water content and local chemical environment. When blood flow is relatively slow, as in the false lumen, some intravascular signal may be produced. The intensity varies according to flow velocity and pulse sequences used, and thus provides a contrast between the true and false lumens.
The complementary modality to spin echo is gradient echo imaging, which produces very bright images in the presence of flowing blood.
The combination of both imaging modalities provides information regarding the relative flow velocities between the fast and slow (true and false) channels and differentiates the origin of aortic side branches between true and false lumens. Maximum intensity projection, applicable to any structure containing flow ing blood, combines images acquired from multiple sections into a pattern similar to that obtained from an angiogram.
The definitive diagnosis of aortic dissection by magnetic resonance imaging requires identification of an intraluminal membrane separating the two channels
. Using spin echo imaging, this is identified easily as a thin linear structure of moderate intensity, contrasted by the dark void from flowing blood in the true and false lumens.
As with other imaging modalities, when the false lumen is thrombosed, this channel may be difficult to distinguish from concentric thrombus in an atherosclerotic aortic aneurysm.
MRI has not gained widespread use for evaluation of acute aortic dissection because imaging is time-consuming and may not be available on a 24-hour basis. Magnetic resonance imaging is most useful for follow-up of chronic dissection since it is reproducible, reliable, and completely noninvasive. Erroneous results from MRI scanning are extraordinarily rare; therefore this is the current gold standard for the diagnosis of aortic dissection.
ECHOCARDIOGRAPHY
Both transthoracic (TTE) and transesophageal (TEE) echocardiography are used to provide rapid diagnosis of aortic dissection. Given increasing availability and operator expertise, echocardiography is the preferred diagnostic imaging method for acute aortic dissection. Echocardiography easily identifies aortic regurgitation, pericardial effusion, and dilation of aortic segments. Transthoracic echocardiography provides limited views of the ascending aorta and because of body habitus may not be applicable in all patients. Transesophageal echocardiography using new omniplane technology provides superb imaging of the ascending aorta, but is semi-invasive and requires sedation to prevent discomfort and hypertension. Echocardiography does not require intravenous contrast agents or exposure to radiation.
With transthoracic echocardiography, the criteria for definitive identification of the dissection include
(1) visualization in more than one view
(2) defined motion not parallel to the motion of any other cardiac structure;
(3) differentiation from an extension or reverberation of another cardiac structure; and
(4) identifiable true and false channels separated by a repeatedly observed echogenic surface (dissection membrane).
The entire thoracic aorta must be visualized, to exclude the diagnosis.
Color flow Doppler velocity mapping improves both sensitivity and specificity of the transthoracic evaluation, quantitates the severity of aortic regurgitation, and demonstrates communications between true and false channels.
Nienaber reported a sensitivity of 78.3 percent for acute and 87.5 percent for subacute proximal dissections and 40 versus 29.4 percent for acute and subacute distal dissections in a large series comparing transthoracic echocardiography to other modalities.
Although aortic regurgitation and pericardial effusions were detected easily, transthoracic echocardiography poorly imaged the aortic arch.
The transesophageal probe, first used for diagnosis of aortic dissection by Borner in 1984, eliminates many of the limitations of transthoracic imaging, including obesity, emphysema, mechanical ventilation, chest wall deformities, and small intercostal spaces.
Current state-of-the-art transesophageal equipment includes omniplane transducers that combine multifrequency two-dimensional (2-D) imaging, steerable pulse wave, continuous wave Doppler, and color flow imaging. The probe rotates a full 180°, to provide almost unlimited viewing planes. Using current transesophageal probes, the thoracic aorta can be examined as well as or better than with any other imaging technique.
TEE also assesses left ventricular function, aortic and mitral regurgitation, regional wall motion, and coronary artery involvement.
Insertion of the transesophageal probe usually requires topical anesthesia and light-to-moderate sedation at the bedside, but is easiest under general anesthesia in the operating room. The examination can be performed in 97 percent of patients in whom it is attempted and has relatively few contraindications. Contraindications include esophageal varices, strictures and tumors, and to a lesser degree, a recent meal or full stomach owing to ileus. Significant complications are rare.
Hemodynamic changes produced during transesophageal echocardiography usually are controllable but are not trivial.Several investigators, including ourselves, have observed aortic rupture during and immediately after transesophageal echocardiography.
In a multicenter study, the accuracy of transesophageal echocardiography was compared to other modalities in 164 consecutive patients suspected to have aortic dissection.
The study correctly identified all DeBakey type I and III dissections and only one false-negative diagnosis occurred in a patient with a localized DeBakey type II dissection. Of note, aortography also failed to identify the dissection in this case. A false-positive echocardiographic diagnosis of DeBakey type II dissection occurred in two patients with dilatation of the ascending aorta.
The combined sensitivity and specificity of TEE were 99 and 98 percent, respectively. When compared to aortography, transesophageal echocardiography was superior.
TEE has the important advantage of positively identifying many of the alternative diagnoses in patients with suspected aortic dissection. TEE reliably detects acute myocardial infarction, pleural effusion, aortic stenosis, pericardial effusion with or without tamponade, and aneurysm of the ascending aorta uncomplicated by dissection.
INTRAVASCULAR ULTRASOUND
Pandian and colleagues used intravascular ultrasound in 14 patients with proven dissection, and correctly identified the entry site, extent of dissection, involvement of the abdominal aorta and its branches, and the presence or absence of clot in the false lumen.
Intravascular ultrasonic probes can detect minuscule aortic wall lesions and may become important for evaluating subtle defects not visible by other techniques.
The method raises safety concerns, poorly images the aortic arch, and does not detect aortic insufficiency or pericardial effusion.
B-MODE ULTRASOUND
B-Mode ultrasound and Doppler ultrasound can evaluate the abdominal aorta and peripheral manifestations of aortic dissection not shown by transesophageal echocardiography.
Diagnostic Strategy
A patient suspected of an acute aortic dissection may require one to three imaging modalities described earlier.
The presence or absence of a dissecting membrane in the ascending aorta must be ascertained with absolute certainty and should be differentiated from intramural hematoma.
Demonstration of a dissection membrane is an absolute indication for operative replacement of the ascending aorta and, in the absence of complicating factors, no further diagnostic evaluation is necessary, as the arch and coronary ostia can be examined during operation. The decision of which imaging modality to employ to demonstrate a dissecting membrane in the ascending aorta depends on clinical urgency, costs, and institutional availability.
In patients with cardiovascular collapse and a high likelihood of acute aortic dissection (i.e., severe chest pain without ECG changes, a new murmur of aortic regurgitation, or pulse deficit) immediate transfer to the operating room without confirmatory imaging is recommended. After careful induction of general anesthesia, the transesophageal echocardiography probe is inserted and an evaluation of the aorta is done at that time. This places the patient in the safest and most useful area of the hospital for patients with acute complicated aortic dissection—the cardiovascular operating room. If the diagnosis proves false, or if the ascending aorta cannot be visualized, clinical stability can be obtained easily and rapidly in the cardiovascular operating room, invasive monitoring lines inserted under sterile conditions, and (with the patient under the best possible control), he or she may be transferred to the angiography suite for further evaluation.
If the patient is clinically stable, the most convenient method for confirming the diagnosis is employed. Often a transthoracic echocardiogram quickly demonstrates an intimal flap in the ascending aorta. If conclusive data are not obtained, the transesophageal probe is inserted or the patient is taken to the CT scanning suite, where ultra-fast spiral CT scanning provides a complete assessment of the aorta in 10 minutes. Magnetic resonance scanning is reserved for occasional patients who are completely stable and have questions regarding the precise location of the intimal tear or involvement of the ascending aorta and aortic arch. Thus, the most common use of MRI in the acute setting is to rule out involvement of the aortic arch in patients with acute distal dissection, since the extent of retrograde dissection into the arch is not well-assessed by other imaging techniques.
The use of angiography for acute aortic dissection continues to decrease for several reasons. Most hospitals do not maintain an angiography team in the hospital on a 24-hour basis. The procedure is painful and may exacerbate hypertension to cause extension or frank rupture. Thrombosis of the false lumen may obscure the diagnosis. If the false lumen is less than one-half of the diameter of the true lumen, the double channel may not be perceived. Finally, contrast material required for satisfactory images may cause nephrotoxicity. Nevertheless, angiography remains the preferred procedure for evaluation of possible obstructed aortic branch vessels.
Natural History
In autopsy reviews, at least 50 percent of patients with untreated acute aortic dissections succumb within the first 48 hours; this yields a 1 percent per hour death risk for acute aortic dissection.
This fact should be kept in mind by all physicians evaluating patients suspected of aortic dissection—time is paramount. In the largest collective review of aortic dissection, comprising 963 patients with both proximal and distal untreated dissection, 36–72 percent of patients succumbed within 48 hours, 71 percent at 2 months, 89 percent at 3 months, and 91 percent at 6 months.
Indications and Selection of Operation
PROXIMAL AORTA AND ARCH DISSECTION
Acute aortic dissection that originates in or extends to the ascending aorta is an absolute indication for emergency operation to prevent rupture into the pericardium. A double lumen in the intrapericardial aorta establishes the indication regardless of the origin of the tear. The false lumen may have flow exceeding that of the true lumen, contain partial thrombus, or be completely thrombosed. The simple presence of two channels in the ascending aorta indicates the need for emergency repair.
The selection of specific operations varies with the pathology. Whenever possible, the native valve is resuspended and replacement is avoided.
The presence of stroke or acute paraplegia does not contraindicate operation, as patients may experience significant recovery after operative repair.
Obviously, in cases of brain death, operation does not proceed.
In the absence of phenotypic or historical markers for a connective tissue disorder, patients with involvement of the ascending aorta and a normal aortic root may undergo simple tube-graft replacement of the ascending aorta, resuspension of the aortic valve, and an open anastomosis with reconstitution of the downstream aorta.
Patients with either historical or phenotypic markers for a connective tissue disorder or who have dilatation of the sino-tubular portion of the aorta or aortic annulus require composite graft replacement of the aortic valve and ascending aorta with reimplantation of the coronary arteries. For patients with a connective tissue disorder (e.g., Marfan syndrome) and a relatively normal aortic valve, the native aortic valve can be preserved with prosthetic replacement of the ascending aorta, including the sinus portion.
At this time, the authors neither recommend for or against this procedure. Regardless of whether or not the valve is replaced, it is important to use a composite prosthesis and reimplant the coronary arteries. This avoids a high incidence of subsequent sinus of Valsalva aneurysm that occurs after simple aortic replacement without resection of the aortic sinuses.
Aortic dissection usually originates in the dilated portion of the ascending aorta in patients with acquired aortic stenosis or a congenital bicuspid valve. In these patients, the sinus portion of the aorta that contains the coronary ostia is retained, the valve is replaced, and the dissected ascending aorta is replaced with a Dacron graft after reuniting the two ends of the transected aorta with either resorcinol glue or Teflon felt.
In selected cases, such as female patients who wish the option of future pregnancy, use of cryopreserved or fresh aortic homografts is advocated for composite replacement of the aortic valve and ascending aorta for acute aortic dissection. Long-term data on the surgical results of this procedure are not yet available.
Some surgeons recommend a “tie-in” prosthesis for the distal connection between graft and dissected aorta.
Theoretically, this technique decreases the incidence of a perfused false lumen as compared to reuniting the dissected layers with a Teflon felt sandwich, but this advantage has not been demonstrated in a large group of patients from multiple centers. Furthermore, the rigid internal ring of the prosthesis reduces intraluminal diameter and may compromise the origin of the innominate artery. The authors prefer an open-sutured anastomosis, either with two layers of Teflon felt or resorcinol glue for reconstitution of the distal aorta.
When the proximal dissection extends into and beyond the aortic arch (DeBakey type I), graft replacement of the ascending aorta is extended into the lesser curvature of the arch.
This facilitates later repair of the arch and thoraco-abdominal aorta if disease of these segments progresses. In patients with DeBakey type II dissections that involve only the ascending aorta, resection removes the entire separated aorta.
The extent of ostial coronary arterial involvement is determined intraoperatively. If the aortic root is intact and there is no intimal tear extending into the coronary artery (i.e., the layers are only separated), the layers of the aortic root can be repaired using either Teflon felt or glue with rare coronary artery problems. If a tear extends into the coronary artery or if the dissection involves either main coronary ostium, a composite graft and repair of the coronary artery using glue is used. The coronary ostia are connected to the graft by direct reimplantation or by the Cabrol intercoronary graft technique. The need to oversew a dissected ostium and perform a distal coronary artery bypass graft is rare.
If the inner cylinder of the aortic arch is not torn, the arch is managed identically to simple ascending aortic dissection. If the arch is torn, the portion containing the tear is resected and replaced using the open anastomotic technique.
More radical operations are required when the intima of the arch is fragmented, the arch is aneurysmal, the outer margin is ruptured, or the brachiocephalic vessels are disrupted. In such cases one or more of the brachiocephalic vessels are directly reimplanted into a Dacron graft after reconstitution with either Teflon felt or glue.
Unusually, dissection may originate in the arch and propagate proximally as far as the aortic root. Even more rarely, a dissection originating in the descending thoracic aorta may propagate retrograde as far as the ascending aorta. Operation for these two situations is similar to dissections originating in the ascending aorta. When the tear originates in the aortic arch, the site must be resected in conjunction with replacement of the ascending aorta. With distal retrograde dissection separating the layers of the ascending aorta, we replace the ascending aorta and reunite the layers of the proximal aortic arch. This converts the retrograde dissection to a DeBakey type III.
DISTAL AORTIC DISSECTION
Patients with distal dissection tend to be older, hypertensive, and have a higher incidence of atherosclerotic disease, which may include other vascular beds such as the coronary and renal arteries. Currently, surgical intervention for acute distal aortic dissection is reserved for complications of the disease and is either not involved or has only a limited role in the management of uncomplicated patients.
Surgery for acute distal dissection remains controversial. Obviously, patients with perforation of the descending thoracic or thoraco-abdominal aorta require urgent operation. However, if the dissection stabilizes without frank rupture, patients may be managed medically in most circumstances.
Medical treatment consists of invasive hemodynamic monitoring, beta blockade, and arterial dilatation that is started during the initial evaluation. Once the patient is stable, oral beta-blockers and antihypertensive medications are substituted and the patient is allowed to return to normal activity with close follow-up. If the patient becomes asymptomatic with medical therapy, good results without surgery are anticipated, particularly if the diameter of the thoracic and thoraco-abdominal aorta is normal or only slightly enlarged.
Medical management of patients with acute distal dissection can be justified because worldwide accumulated experience shows that medical therapy is effective for preventing death in acute distal dissections.
Furthermore, operative mortality and complication rates are much higher in acute distal aortic dissections. Nevertheless, long-term complications of acute distal aortic dissection eventually cause death in up to 84 percent of individuals treated medically. Thus, these patients require extremely close follow-up throughout the early and late post dissection period. Centers following large numbers of medically treated patients after acute distal dissection report mortality rates ranging 21–67 percent with an interim term survival of 30–40 percent.
A significant number of early deaths occur following aortic rupture or after attempted repair of extensive aneurysmal enlargement.
Indications for operation in patients with acute distal dissection generally are limited to prevention or relief of life-threatening complications. These complications include aortic rupture, ischemia of limbs and organ systems, persistent or recurrent intractable pain, progression of the dissection, and uncontrolled hypertension.
Operation for acute distal aortic dissection is tailored to the indication for operation. In cases of intractable pain, without evidence of frank rupture, we replace the proximal portion of the descending thoracic aorta beginning above the dissection at the left subclavian artery and extending through the proximal one-third to one-half of the descending thoracic aorta. This procedure eliminates the most likely site of aortic rupture and is unlikely to interfere with the blood supply to the spinal cord. When operating for a ruptured descending thoracic or thoraco-abdominal dissection, replacement generally begins at the distal aortic arch, hopefully to a non-dissected proximal segment, and is carried distally to include any segment of the disrupted aorta. Preservation of the blood supply to the spinal cord and viscera must be included, if necessary, in this repair.
In most series, operations for acute distal aortic dissection carry a higher mortality, that historically ranges between 35–75 percent, than for acute proximal dissection. This is primarily because these patients usually are operated on under dire circumstances when medical therapy fails.
More recent series show some improvement in mortality rates, but they remain elevated, particularly in patients with preoperative visceral ischemia owing either to a malperfusion syndrome or excessive anti-hypertensive therapy. In the Stanford series mortality increased from 23 to 80 percent in the presence of renal or visceral ischemia, and from 21 to 71 percent in the presence of frank rupture of the aorta.
Age also played an important role in this series; 60 percent of patients over 70 died, whereas only 10 percent of those under 40 succumbed.
In summary, patients with acute distal aortic dissection are managed medically with surgical intervention reserved for complications. The more common indications for operative intervention include persistent or recurrent pain, ischemia or infarction of major end organs, progression of the dissection during medical treatment, and frank or contained aortic rupture. There is no absolute contraindication for operative intervention, but expected mortality may be prohibitively high in patients with significant co-morbidity that includes lung disease, ischemic cardiac disease, advanced age, and renal and visceral ischemia. Acute paralysis is not a contraindication to operation, particularly if femoral pulses are absent, as often relief of distal ischemia returns motor and sensory functions.
CHRONIC AORTIC DISSECTION
Aortic dissection becomes chronic in survivors of asymptomatic acute dissections, those successfully treated medically, and in unresected dissected aortic segments in survivors of acute aortic dissections. Chronic aortic dissection usually is asymptomatic although mild, dull chest pain from congestive failure may be present.
Large post-dissection aortic aneurysms may cause severe skeletal pain from erosion of the bony thorax.
Most operative interventions for chronic thoraco-abdominal dissections are prompted by aneurysmal enlargement. Probably because of distal re-entries that decompress the false lumen, pulse deficits are unusual.
Rarely, chronic aortic dissection causes paralysis or paraplegia following thrombosis of a false lumen from which significant intercostal arteries originate. Embolic strokes may occur from thrombus in proximal pockets of the false lumen.
The selection of diagnostic imaging modalities is wider in patients with chronic aortic dissections. The primary indications for imaging include periodic surveillance (usually at 6-month intervals), onset of symptoms that may be caused by the dissection, and preparation for operative intervention. Occasionally all four imaging techniques are used, but usually only one or two suffice. CT scanning is a cost-effective method for surveillance in most patients because echocardiography does not image the abdominal aorta well. In many patients, transthoracic echocardiography provides satisfactory surveillance of the ascending aorta. Magnetic resonance imaging often produces spectacular images of intra- and extra-luminal aortic anatomy, and provides precise views of complex areas for planning operative strategy. Aortography is used selectively and may be used to demonstrate branch vessels by selective injection. Unlike acute dissections, selective angiography is safe in patients with chronic dissections. Many patients require coronary angiography prior to elective operations for aneurysmal enlargement of a dissection.
Occasionally a proximal dissection may go unnoticed for weeks, months, or years. These asymptomatic or minimally symptomatic dissections usually occur in a previously dilated aortic root. Surgical intervention is recommended for dilatation of the aortic root beyond 5.5 cm and/or moderate to severe aortic regurgitation.
Operation for chronic distal dissection is prescribed primarily for relief of aneurysmal expansion, and threatened, or leaking ruptures of the dissected segment. Rarely, patients develop chronic visceral ischemia owing to a malperfusion syndrome. Indications for operation are similar to those of other thoraco-abdominal aneurysms, but the risk of spinal ischemia is higher as compared to that seen with atherosclerotic aneurysms.
The authors advocate replacement of the descending thoracic and thoraco-abdominal aortic segments either by a single or multiple stage operation when the affected segment reaches two times the normal aortic diameter.
Factors that increase the likelihood of rupture include eccentricity of an aneurysmal segment and rapid expansion. In addition, several reports show that smoking may dramatically increase the risk of rupture. Replacement of any segment of aorta that becomes symptomatic, is tender to palpation, or productive of chronic back pain is recommended. If the aneurysm grows more than 1 centimeter during a single 6-month interval, operation is indicated.
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